To accurately measure atmospheric conditions such as greenhouse gas concentrations, sensors must be pointed toward cloud-free “open sky” areas. For example, a downward looking sensor that passively detects spectral content associated with greenhouse gases can be mounted on an Earth-orbiting satellite. Using a mirror system or the like, the line of sight of the sensor can be steered over a range of angles so that measurements can be taken in a number of different directions within a region of the Earth's atmosphere.
Conventionally, the process of taking sensor measurements in several directions within a region has been performed without knowing whether clouds obscure the sensor's field of view (FOV) in any of the directions. Only after later evaluating a contemporaneously generated visual image of the region can it be determined whether the field of view of each of the measurements was in cloud-free areas and therefore usable. FIG. 1 shows a conventional method of scanning a region to detect greenhouse gas concentrations. Within the region, a sensor is sequentially pointed to twelve angular locations (e.g., at regular intervals in a two-dimensional raster scan), labeled 1 through 12, to collect information. At each location, the sensor is configured to detect greenhouse gas concentrations in a FOV area, represented by the circles in FIG. 1. As shown in FIG. 1, out of the twelve locations, the sensor is able to accurately sense greenhouse gas concentration only at location 10 (shown in dashed lines), since the FOV area at location 10 is free of clouds. As a result, only one out of twelve measurements collected by the sensor can be used in this example. Because the sensor system is unaware of the presence or location of cloud cover, the system cannot steer the sensor angle to specific locations to take advantage of significant open-sky areas that may exist in the region. Once the entire region in FIG. 1 has been scanned by the sensor, the sensor is moved to the next region to repeat the scanning process. Despite the considerable time required to sequentially scan a number of regions of interest, many of the measurements may not be useable due to cloud cover, and the success rate of collecting useful data is low.
To improve the success rate, a smaller FOV may be used. And to reduce the time for scanning, a plurality of sensors can be employed to scan a region. FIG. 2 shows another conventional method of scanning using eight sensors disposed in a row. In operation, the eight sensors produce eight FOVs in a row, represented by eight circles in one row. The row of FOVs are moved in a direction represented by an arrow in FIG. 2 to scan the region. Because eight sensors are employed to scan the region, the time required to scan the entire region is reduced. Further, because a smaller FOV is used, more areas (in the broken-line circles) not covered with cloud can be used to detect gashouse gas concentrations, improving the number of useful measurements within a region. However, the use of smaller FOV results in lower signal to noise ratios, and hence lower-quality measurements, and the method still collects data that cannot be used for the purpose of detecting gashouse gas concentrations.